In order that an enormous amount of user information may be communicated between a wireless network comprising at least a plurality of base stations, and a large number of mobile stations present in one or more cells formed by each of the base stations, a cellular system employs the same frequency carrier (simply referred to as carrier hereinbelow) in a plurality of cells at the same time.
In such a cellular system, to make use of the same carrier in as many cells as possible at the same time, there has been proposed a method involving dividing a cell into an area closer to the base station (referred to as a cell center hereinbelow) and an area farther from the base station (referred to as a cell edge hereinbelow) as shown in FIG. 1; allocating a predetermined carrier (F) to the cell center of all cells for use at the same time; and dividing the remaining carriers into three groups (f1, f2, f3), for example, in the cell edge and allocating each group to every third cells for use, for preventing simultaneous use of one group in mutually adjacent cells (see Patent Document 1, for example).
To achieve such use of a carrier, a base station transmits a common pilot signal at a predetermined level of transmission power in its cell, and a mobile station receives the common pilot signal from a plurality of nearby base stations and measures the levels of the received power. A cell having the highest level of the received power is regarded as a cell of the mobile station's own for communication, and a cell having the second highest level of the common pilot signal is regarded as an adjacent cell, where the mobile station uses the carrier of the cell center when the ratio of the received power level of the common pilot signal for its own cell and that for an adjacent cell is equal to or greater than a predetermined threshold value (referred to as a cell center threshold value hereinbelow). Otherwise, the mobile station uses the carrier of the cell edge. Alternatively, rather than using the ratio of the received power levels of the common pilot signal, a method of using a carrier of the cell center when reception quality of the common pilot (e.g., the received power level of the common pilot signal, or the ratio of the power level of the received common pilot signal and that of an interference signal) for its own cell is equal to or greater than a predetermined threshold value may be contemplated.
Thus, a cellular system using carriers in a plurality of cells at the same time should use as many carriers as possible at a higher utilization rate in as many cells as possible so that the whole system has improved spectral efficiency and increased traffic capacity.    Patent Document 1: JP-1994-311089A